Field of the Invention
The present relates to a process for incorporating of natural fiber and starch into thermoplastics and the natural fiber/starch thermoplastic composite produced.
Description of the Prior Art
Thermoplastic starch is potentially a low cost alternative biodegradable plastic that is a readily available material, but owing to its poor mechanical properties and high susceptibility to water, its actual replacement of the polymers currently in use is limited. As one of the most abundant natural resources, natural fibers have been studied to enhance traditional petroleum based polymers in the past decades. However, due to the hydrophilic behavior of wood fiber, it is challenging to compatibilize polymer matrices with wood fibers and to uniformly disperse wood fibers into polymer matrices with minimal damage. The compatibility between natural fiber and thermoplastics is so poor that natural fiber tends to form agglomerates in thermoplastics which act as stress weak points resulting in decreased mechanical properties. For conventional manufacture of natural fiber/polymer composites, natural fiber should be dried very well prior to compounding, but the feeding of fluffy and entangled fibers into typical polymer process equipment is a great challenge.
Synthetic thermoplastics, such as polyethylene (PE), polypropylene (PP), polystyrene (PS) and poly (vinyl chloride) (PVC), have been widely used in modern society. To avoid the problems associated with the application of synthetic thermoplastics, for example, long-term environmental pollution and high raw material cost, composites utilizing biomaterials have been intensively studied and utilized in numerous applications. Natural fibers including wood fibers are normally used as fillers to reduce cost; however, the incorporation of natural fibers into thermoplastics usually impairs mechanical properties mainly due to the incompatibility between the hydrophobic polymer matrix and the hydrophilic natural fibers as well as fiber damage.
One of the widely used thermoplastics is polypropylene (PP). It is a widely used in the world due to its good electrical insulating properties, chemical inertness, moisture resistance and decent mechanical properties. However, the industries consuming PP always suffer from its high cost, especially at a time of rising petroleum prices. Starch granules and thermoplastic starch (TPS) have been blended with PP in efforts to obtain new materials with low cost and high biodegradability. Starch granules have been directly used as organic filler in PP matrix, with the results showing that with increasing starch granule content the tensile strength of the composites decreased owing to the poor compatibility between the hydrophilic starch and the hydrophobic PP [Roy, S. B., et al., Polypropylene and potato starch biocomposites: Physicomechanical and thermal properties. Journal of Applied Polymer Science, 2011. 120(5): p. 3078-3086]. To improve its compatibility with polyolefins and processability, starch granules were plasticized prior to blending with polyolefins. However, the stress at break of the TPS/PP blends still decreased with increasing TPS content [Kaseem, M., K. Hamad, and F. Deri, Thermoplastic starch blends: A review of recent works. Polymer Science Series A, 2012. 54(2): p. 165-176.]. A scanning electron microscopy (SEM) study confirmed the mechanical results: poor adhesion and interfacial interaction between PP and TPS in the prepared blends was observed.
Natural fibers, such as wood, flax, ramie, jute and commercial regenerated cellulose fibers, have also been blended with polypropylene as a reinforcement and substitute material. These fibers are renewable and abundant in nature; therefore, the cost of the natural fibers is much cheaper than polypropylene. Besides, the natural fibers are non-abrasive so that relatively large concentrations could be incorporated into polyolefins without causing serious machine wear during fabrication [Woodhams, R. T., G. Thomas, and D. K. Rodgers, Wood fibers as reinforcing fillers for polyolefins. Polymer Engineering & Science, 1984. 24(15): p. 1166-1171]. There is also increasing demand for light-weight, cost-effective, green and sustainable composite products and blends. Compared to traditional reinforcing glass fiber, natural fiber has lower density, higher specific strength, and improved health and safety in handling. Wood fiber requires up to 60% less energy to produce and is carbon neutral. The global plastics market was estimated around 300 million tonnes in 2010, of which the market for glass fiber as reinforcement in structural composites—primarily automotive, packaging, construction—is estimated at 4-5 million, with potential annual growth of over 6%.
Usually, the incorporation of more than 50 wt. % natural fiber is desired for industrial scale fabrication in order to minimize product cost. However, it should be noted that with increasing natural fiber fraction, some properties of the composites decrease dramatically. For instance, the tensile and impact strength of thermo-mechanical pulp reinforced PP composites decreased from around 30 MPa and 51 J/m to around 14 MPa and 31 J/m when the fiber content increased from 0 to 60 wt. %, respectively [Mantia, F. P. L., M. Morreale, and Z. A. M. Ishak, Processing and mechanical properties of organic filler—polypropylene composites. Journal of Applied Polymer Science, 2005. 96(5): p. 1906-1913]. The property reduction is because of poor dispersion in the matrix and weak interfacial adhesion between fiber and matrix as well as fiber damage [Bledzki, A. K., S. Reihmane, and J. Gassan, Thermoplastics Reinforced with Wood Fillers: A Literature Review. Polymer-Plastics Technology and Engineering, 1998. 37(4): p. 451-468]. The tendency of fibers to self-agglomerate, especially for the fibers containing more than 10 wt. % moisture, makes it difficult to disperse uniformly in a hydrophobic matrix. Currently, natural fibers, prior to blending with polyolefins, must be dried to less than 1 wt. % moisture in order to reduce fiber self-agglomeration [Karmarkar, A., et al., Mechanical properties of wood-fiber reinforced polypropylene composites: Effect of a novel compatibilizer with isocyanate functional group. Composites Part A: Applied Science and Manufacturing, 2007. 38(2): p. 227-233.], which requires drying equipment and consumes a lot of energy. Pelletizing fiber is another approach to facilitate fiber feeding and dispersion. The pelletization process includes increasing fiber moisture content to 60˜70%, pelletizing with a mesh and a rotating knife, and drying to less than 1% moisture, which obviously increases the cost and cannot avoid fiber damage. For example, the length and aspect ratio of chemi-thermomechanical pulp fibers was reduced from 1.50 mm and 42 to 0.84 mm and 23.9, respectively, after pelletization [Nygard, P., et al., Extrusion-based wood fibre-PP composites: Wood powder and pelletized wood fibres—a comparative study. Composites Science and Technology, 2008. 68(15-16): p. 3418-3424]. Severe fiber damage was normally observed during extrusion. For example, La Mantia et al. [ibid] reported that after twin-screw extrusion, the length of the wood fibers in a polypropylene composite containing 60 wt. % fibers decreased by more than 80%. It is still a big challenge to uniformly disperse natural fibers in polymer matrices with minimized fiber damage.